Emerging solutions for Wavelength Switched Optical Networks (WSONs) will simultaneously allow mesh connectivity of multiple bitrate channels over the same optical transmission infrastructure. In this way it will be possible to provide a flexible transport platform for current and future bitrates. It means that when upgrading existing telecommunications and data networks to operate as WSON a provision of 40 Gbit/s (or even at higher bitrate) optical signals in conjunction with existing 10 Gbit/s signals will be necessary.
In a multi bitrate WSON scenario, Cross-Phase Modulation (XPM) is particularly detrimental on Phase-Shift Keying (xPSK) modulated signals when induced by On Off Keying (OOK) modulated signals. As an example, a 10 Gbit/s lightpath, which is OOK modulated, may induce a very detrimental XPM on a 40 Gbit/s lightpath, which is typically Differential Quadrature Phase-Shift Keying (DQPSK) modulated. Indeed, the changes on the fibre refractive index due to the intensity modulation of an OOK cause perturbation of the xPSK signal phase. In turn, the effects of a 40 Gbit/s lightpath on a 10 Gbit/s lightpath are not so detrimental, as well as between lightpaths at the same bitrate.
In known solutions, XPM is considered as a penalty (e.g., on Optical Signal to Noise Ratio—OSNR), computed in the worst-case scenario. In the case of multi bitrate WSON, the worst-case scenario consists of a xPSK modulated lightpath, which is positioned at the centre of the Dense Wavelength Division Multiplex (DWDM) comb of OOK modulated wavelengths. In this way, if the Quality of Transmission (QoT) of the xPSK modulated lightpath is acceptable, accounting for the worst-case penalty, the QoT is acceptable with any other arrangement of lightpaths within the DWDM comb.
The biggest disadvantage of the known method is that the QoT evaluation accounting for worst-case penalty is pessimistic in the way that some wavelengths are classified as unacceptable for lightpath assignment even though in real life network there would be no problem with operating lightpaths on them. Indeed, a set of paths is considered unacceptable in terms of QoT (i.e. the resulting Bit Error Rate—BER—is considered unacceptable) even if the worst-case scenario is not present. Upon lightpath request, the Path Computation Element (PCE), which computes the path to provision the lightpath, has to exclude some actually acceptable paths, resulting in inefficient resource utilization.